Synthesis mechanism of double perovskite Sr1.9K0.1FeMoO6 by sol-gel method

Zhai Yongqing, Zhang Zhang, Feng Shihua, Huo Guoyan, Shi Lingjuan, Hao Xiaohong
(College of Chemistry and Environmental Science, Hebei University, Hebei Baoding 071002 )

Abstract The sol-gel method is used to prepare double perovskite oxide Sr1.9K0.1FeMoO6. In this paper, the products at different stage during the process of Sr1.9K0.1FeMoO6 synthesis were analyzed by X-ray diffraction, infra-red spectra, thermal gravimetric and differential thermal, and X-ray energy disperse spectra. The results reveal the primary mechanism of sol-gel method: component elements still exist as the form of amorphous citric complexes after the pyrolysis of gel. Then the precursor after pyrolysis is transformed into simple compounds SrMoO4 and SrFeO3-x. Finally, reduction reaction leads to the formation of Fe-O-Mo bond, i.e., generating the final product Sr1.9K0.1FeMoO6.

Ten years after the discovery of giant magnetoresistance (GMR), the observation at room temperature of a large magnetoresistance in magnetic tunnel junctions gave rise to an increase of the interest for these systems mainly due to their potential applications such as recording media, field sensors or heads for hard drives [1, 2].
The double perovskite Sr2FeMoO6 has been brought recently into the center of scientific interest because of its considerable magnetoresistance observed already in the relatively low magnetic fields even at and above room temperature [3-6], which offers potential applications in spintronic devices and is of fundamental interest in condensed matter physics.
As reported by Tovar et al. [7], the strength of magnetic coupling is directly related to the density of electrons at the Fermi level. Therefore, in order to enhance the TC of Sr2FeMoO6, injection of electrons into the conduction band by appropriate doping is a natural strategy. Alkaline earth metal substitution and rare earth substitution for Sr are widely studied based on the effect of isoelectronic doping and electronic doping [8-12], while the alkali metal substitution is hardly reported.
The compounds Sr2FeMoO6 reported previously were prepared by using the method of traditional solid state reaction, which is carried out at a higher sintering temperature, and therefore only the polycrystalline materials with a large grain size were produced. Band calculations indicated Sr2FeMoO6 had a half-metallic electronic state and its MR has been assumed to be dominated by spin-polarized tunneling through insulating grain boundaries [3]. Its smaller grain size leads to larger MR effect at low applied field, so large grains are not beneficial to enhance the low field IMR (intergrain tunneling magnetoresistance) [13]. There are several advantages on sol-gel method; homogeneous mixing can be attained in relatively low temperature, and small grain size can be expected on the products and so on.
In this paper, the alkali metal K-doped Sr2FeMoO6 was synthesized by sol-gel method with active carbon as reducing agent, and primary mechanism of sol-gel method is revealed by analyzing the products at different stage during the process of Sr1.9K0.1FeMoO6 synthesis.

2.1 Synthesis of Sr1.9K0.1FeMoO6

Polycrystalline Sr1.9K0.1FeMoO6 samples were prepared by Sol-Gel method. Stoichiometric powders of Sr(NO3)2, Fe(NO3)3·9H2O and (NH4)6Mo7O4·4H2O were mixed with citric acid and then dissolved. The pH of the solution was adjusted by aqueous ammonia. A slight green gel was formed after several hours under the condition of water bath. Then the gel was put into oven and dried at 90¡ãC. Subsequently, the dry gel was pyrolyzed at 180¡æ. The dry gel was frothed, fumed, and finally formed a dry black sponge (which was called precursor). Then the precursor was pre-sintered at 600¡ãC. Finally, the powder was mixed with appropriate active carbon powder and then sintered in the reducing atmosphere provided by active carbon particles to obtain the final product.
2.2 Characterization
Differential thermal analysis (DTA) and thermogravimetric (TG) analysis were carried with a DT-40 analyzer, at a rate of 20¡ãC/min under static air conditions in the temperature range of 20 to 735¡ãC, to analyze the decomposition reaction and phase transformation of the precursor. I.R. spectra were recorded on a Nicolet 380 spectrometer (400-4000 cm-1, KBr pellets). The crystal structure and the phase purity of the samples were examined by X-ray powder diffraction (XRD) using a Y2000 diffractometer with Cu Ka radiation (30kV¡Á20mA) at room temperature. XRD was performed on powdered samples over the 2q range of 15¡ã to 75¡ã. A step scan mode was employed with a step width of 2q = 0.06¡ã and a sampling time of 1s. The size and morphology of the samples were investigated with a KYKY-2800B scanning electron microscope (SEM). The content of the cations were analyzed by X-ray energy dispersion spectrometer (EDS). The field dependence of the magnetization was recorded by a vibrating-sample magnetometer (WKVSM) in a field of 500Oe.

3.1 TG-DTA analysis

The TG-DTA curves of the precursor are shown in Fig.1. It shows that there is a little weight loss in the stage of 50~250¡ãC in the TG curve, which is due to the dehydration of the precursor. The weight loss mainly occurs in the stage of 250~620¡ãC and has little change after 700¡ãC. Two small exothermic peaks at 422¡ãC and 497¡ãC in DTA curve can be assigned as the thermal decomposition of NH4NO3 and free citric acid. The large composite exothermic peak in the stage of 500~620¡ãC can be ascribed to the thermal decomposition and oxidation of the citrate complex. Thermal effect is not distinctive after 700¡ãC, which indicates that the complex has decomposed completely.

Fig.1 TG-DTA curves of precursor obtained after pyrolysis

3.2 XRD analysis
The XRD pattern of the precursor obtained after pyrolysis under 180¡ãC is showed in Fig.2. From Fig.2, no obvious diffraction peak can be observed, which indicates that the precursor is amorphous.
The XRD pattern of the product obtained after pre-sintering under 600¡ãC is showed in Fig.3. Through comparing Fig.2 with Fig.3, it can be seen that the precursor has decomposed after pre-sintering and formed the mixture of SrMoO4 and SrFeO3-x. A little amount of SrCO3 is found in this product, which may results from the CO2 in the air during the process of the pre-sintering.

Fig.2 XRD pattern of precursor obtained after pyrolysis

Fig.3 XRD pattern of product obtained after pre-sintering

The XRD patterns of Sr1.9K0.1FeMoO6 and Sr2FeMoO6 are showed in Fig.4. It shows that Sr1.9K0.1FeMoO6 has the same double perovskite structure as Sr2FeMoO6. All the reflections can be indexed based on the space group I4/mmm by the Jade5 program and the appearance of the superstructure reflections (101) and (211) at 19¡ã and 37¡ã indicates the ordering of the Fe and Mo atoms in our samples.

Fig.4 XRD patterns of Sr2FeMoO6 and Sr1.9K0.1FeMoO6

From the data in Fig.4 and Table 1, it can be seen that the partly substitution of K+ for Sr2+ in Sr2FeMoO6 results in the diffraction peaks slightly shift to large angle, while the crystal plane index has no significant change and the unit-cell parameters decrease slightly. This can be explained by two factors: one is the steric effect; the other is the electron-doping effect. On the one hand, the cell volume of the compounds will be increased because of the substitution of bigger alkali metal ions for comparing the data of ionic radius: r(Sr2+) = 112pm, r(K+) = 138pm. On the other hand, as reported in Refs. [14], in the rare earth doped double perovskite compound Sr2-xLaxFeMoO6, electron doping can increase the unit-cell volume of the compounds because the extra electron provides delocalized carriers selectively to the spin-down t2g metallic spin channel, which will increase the density of states at the Fermi level. These carriers can modify the bond lengths through screening of the ionic potentials that determine the interatomic distances. For the as-synthesized Sr1.9K0.1FeMoO6, the doped K+ is bigger than Sr2+ but has fewer electrons, which may lead to the opposite electron-doping effect, i.e. the doping of K+ will reduce to the the decrease of cell volume. Thereby, for the sample Sr1.9K0.1FeMoO6, the electron-doping effect is slightly stronger than the steric effect, which results in the decrease of the cell volume.

Table 1 XRD data and unit cell parameters of Sr2FeMoO6 and Sr1.9K0.1FeMoO6

parameters (112) (004) (312) a(Å) c(Å) V(Å3)
Sr2FeMoO6 32.096¡ã 46.014¡ã 57.139¡ã 5.580 7.882 245.4
Sr1.9K0.1FeMoO6 32.121¡ã 46.073¡ã 57.243¡ã 5.573 7.872 244.5

3.3 IR analysis
The IR spectrums of the precursors obtained after pyrolysis, pre-sintering and the final product are showed as curve a, b and c in the Fig.5. In the curve a, the absorption peaks at about 854cm-1 and 1384cm-1 are due to the stretching vibration of NO3; the absorption peak at about 1614cm-1 is due to the stretching vibration of COO in the citrate complex; the sharp absorption peak at about 2170cm-1 is due to the stretching vibration of CO2 as the fluffy precursor may absorb a little CO2; the wide and smooth stretching vibration band at about 3420cm-1 is ascribed to the intermolecular hydrogen bond or the O-H in the water molecular. Through comparing curve a and b, it can be found that the stretching vibration bands of NO3, COO, O-H and CO2 weakened obviously after the pre-sintering; the stretching vibration bands at about 1457cm-1, 830cm-1 and 618cm-1 are ascribed respectively to CO32-, MoO42- and Fe-O in the FeO6 octahedral of perovskite structure, which is consistent with the result of XRD analysis (in Fig.3). The curve c shows that the stretching vibration bands of NO3, COO, O-H, CO2, CO32- and MoO42- in final product almost disappear; the new absorption band at about 480cm-1 can be ascribed to the bending vibration of Mo-O-Fe, which indicates that the Mo-O and Fe-O bonds has transformed into Mo-O-Fe bonds after the thermal reduction.

Fig.5 IR spectra of precursors obtained after pyrolysis, pre-sintering and the final product

3.4 EDS analysis

Fig.6 EDS pattern of Sr1.9K0.1FeMoO6

    Fig.6 shows the X-ray energy dispersion spectrum (EDS) of the final products. Based on it, the molar ratio of Sr, K, Fe and Mo in this sample obtained by quantitative analysis is nearly agreement with the designed ratio (1.9 : 0.1 : 1 : 1), which indicates that the composition elements of the raw material have transformed into final product and the final product is expected Sr1.9K0.1FeMoO6.
3.5 Magnetism of sample
The magnetization versus field curve (M-H) of Sr1.9K0.1FeMoO6 is showed in Fig.7. It shows that the magnetization was still not saturated as the applied field rose to 0.7 T from 0 T. The saturation magnetization of the sample is 1.62mB/f.u., which is obtained as the linear part of magnetization versus 1/H curve was extrapolated to 1/H = 0.

Fig.7 M-H curve of Sr1.9K0.1FeMoO6

The primary mechanism of sol-gel method for the synthesis of Sr1.9K0.1FeMoO6 were obtained by the means of X-ray diffraction, infra-red spectrum, thermal gravimetric and differential thermal, and X-ray energy disperse spectrum. It is as follow: component elements still exist as the form of amorphous citric complexes after the pyrolysis of gel at 180¡ãC and no corresponding oxides are formed; then the precursor obtained after pyrolysis is decomposed and transformed into simple compounds SrMoO4 and SrFeO3-x by pre-sintering at 600¡ãC; finally, thermal reduction reaction at1200¡ãC leads to the formation of Fe-O-Mo bond, i.e., generating the final product Sr1.9K0.1FeMoO6.